The Karabiner Model 1931 (K31) is a magazine-fed, straight-pull bolt-action rifle. It was the standard issue rifle of the Swiss armed forces from 1933 until 1958, though examples remained in service into the 1970s. It has a 6-round removable magazine, and is chambered for the 7.5x55mm Swiss (also known as Gewehrpatrone 1911, GP11, or unofficially 7.5x55mm Schmidt Rubin), a cartridge with ballistic qualities similar to the 7.62x51mm NATO/.308 Winchester cartridge. Each rifle included a 6 round detachable box magazine with matching engraved serial number. A stripper clip loads the magazine from the top of the receiver.

The often quoted but incorrect name of "Schmidt-Rubin" comes from two designers: Rudolf Schmidt, who designed the action for Switzerland's 1889 and 1896 rifles, and Lt. Col. Eduard Rubin,[1][2] who designed the ammunition.

Although the K31 is a straight-pull carbine like many other Swiss rifles, it was not designed by Rudolf Schmidt (1832–1898) as he was not alive to do so. The K31 was a totally new design by Eidgenossische Waffenfabrik in Bern, Switzerland under Colonel Furrer, and the gun does not have the Schmidt-designed 1889 or 1896 action. The first 200 K31s were made in May 1931 for troop trials (serials 500,001 - 500,200), thus the model number of 1931.

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The K31 is noted for its straight-pull action, meaning that the bolt is pulled directly back, then pushed forward to cycle the action between shots, rather than being turned and pulled back and forth, as in the Mosin Nagant pattern rifles such as the M1891, or as in the more well known Mauser pattern rifles such as the K98k.

K31s are also noted for their excellent accuracy and quality. The Swiss considered individual marksmanship to be of utmost importance. Therefore, the K31 was made with tight tolerances and excellent overall craftsmanship. Many shooters are able to achieve one minute of arc with unmodified K31s. This means that a group of bullets shot at 100 yards will stay within a 1" diameter area, a group at 200 yards will stay within 2", etc. This is achievable with factory sights. Clamp-on sighting options for scopes and competition sights make it easier to mount a scope on the receiver.

K31s use a unique formed phenolic resin embedded paper charging clip with a tinned metal edge holding six rounds. Whereas most charger clips only hold the end of the round, the K31 charger nearly covers the entire cartridge. The clip has a guide slot wide enough for a gloved thumb to force rounds down and into the magazine in one smooth motion.

Many collectors of the K31 have removed the butt plate and recovered a small tag of plasticized paper from beneath it. This slip contains the name and address of the Swiss citizen to whom the rifle was issued. In some cases, collectors have used the information to contact the previous owners, and have recounted the details of those encounters on a variety of collector's web forums.

The poor condition of many stocks was caused mostly by the hobnail shoes and rifle drills that were common. Postwar beech stocks are more affected than the older walnut ones. Walnut stocks were treated with linseed oil and beech stocks got a shellac protective layer that easily dissolves in alcohol for (arsenal) repairs. Some of the older stocks are reported with both linseed and shellac.

The standard sights on a K31 are open sights that can be adjusted for both windage and elevation. The rear sight is graduated from 100 up to 1500 meters in 100 meter increments. The sight line can be adjusted with a front sight adjustment tool. Moving the front post 1 mm horizontally results in a 12 cm shift at 300 m. To adjust the average height of the point of impact 5 front posts ranging from 5.9 to 7.1 mm height in 0.3 mm increments are available.[citation needed] The change in impact height from one front site to the next is 16 cm at 300 m. Starting at 300 meters the shooter should aim just below the bottom of the target, so that the front sight's post is out of the way.[3] Mounting a scope conventionally is not easily done because of the design of the action, but there are specialized scope mounts available.[4]
As the Swiss still have a militia army where soldiers sometimes keep their rifles for a lifetime many aftermarket sights were available, Waffenfabrik Bern made the "S" and "K" (Klammer) diopter sights, Wyss makes the "W" diopter and Furter, Haemmerli and Gruenig and Elmiger made now rare special windage and elevation fine-correctors, Sahli and many other made elevation fine correctors and these days a company by the name of Swiss Products in the USA makes a clamp-on diopter which was recently approved for use at official Swiss shooting matches.

As a standard service rifle of the Swiss armed forces, the K31 was replaced by the SIG 510 in 1958. As of 2006, the K31 is readily available from most military surplus vendors. As noted above, the stocks are usually in average condition, but the barrel and bolt assembly are usually in very good condition because[citation needed] the Swiss used a special gun grease known as Waffenfett instead of gun cleaning oil.
As of 2010 the Swiss arsenals are long sold out and the rifles now available for sale in countries around the world are ex-Swiss-civilian owned rifles.